Curious cave that stays a cool 56 degrees all year long.

A massive underwater sinkhole made famous by Jacques Cousteau.

Five mile wide circular ring of rock in the Russian Far East is a curious geological oddity.

The coldest city in the world.

The largest dam in the world and the site of a potential disaster.

The convergence of two currents makes this a surprisingly pleasant getaway in the Arctic Circle.

This remote lake in Siberia supposedly contains a fearsome sea monster.

This network of conjoined cellars was riddled with traps and supposedly allowed the townspeople to pretend their homes were deserted.

A three-centuries-old crypt in the foothills of an old plantation.

This peculiar lake of pitch is so thick and viscous that one can walk on its surface.

A lake with hundreds of ancient skeletons surrounding it. The surprise is what killed them.

The villagers of the coldest inhabited place on Earth adjust to unique challenges presented by day-to-day living.

For those with no fear of heights: a skywalk 4,700 feet up on a Chinese mountain.

This vertical pinnacle in China was the inspiration for the floating mountains in the movie “Avatar.”

The second-deepest known cave on Earth, hidden in an extremely remote mountain region.

This ancient burial mound is accessed by an impressive Greek-built archway.

The saltiest lake in all of Crimea is also the pinkest of the pink and teeming with life.

Deadly scalding and one of the largest hot springs in the world.

This stunning mass of ice spans 121 square miles and is still growing.

One man has installed bathtubs into the Utah desert floor to harness the powers of a natural hot spring.

This little known mountain has claimed four times as many lives as Everest.

Abandoned for decades, trains still pass through the now-restored remains of one of Madrid's first metro stations.

Museum design and public transportation meet in this unique Russian train car.

This magnificent blue mosque in Afghanistan is surrounded by thousands of white doves.